This invention relates to a wind-propelled craft of the kind in which the mast is unstayed and supported by the crew. Water-craft of this kind are commonly termed "sailboards" and the present invention will be particularly described in its application to sailboards of improved performance.
In this specification there will be many references to sailboards and attention is directed to British Pat. Nos. 1,551,426 and 1,258,317 for background information on the construction of conventional sailboards.
The maximum speed of a craft can be increased only by increasing the propulsive force acting on it and/or reducing the resistance to forward motion which it experiences. The conventional sailboard configuration has inherent limitations which make significant improvement of either of these aspects impossible. The present invention is directed to the attainment of higher maximum speeds by avoiding (or reducing the effect of) these limitations.
In conventional sailboards travelling at high speed, the craft is subjected to a large headwind component which means that the sail must be trimmed close to the longitudinal axis of the board. Consequently, only a relatively small (propulsive) component of the total sail force acts forward, and a much larger component acts across the board, tending to roll it over. With a conventional sailboard the rolling moment of the sail force can only be counter-balanced by the crew using his weight by leaning backwards (to windward). This sets an absolute limit to the rolling moment which can be balanced since the weight of a given crew is fixed, as is the maximum moment arm on which it can act. Consequently, for any given set of conditions, the maximum propulsive force attainable is also limited by this factor.
Furthermore, as the limit is approached, the proportion of the sail force directed forward is reduced since the sail force vector is tilted upwards. This follows from the fact that the crew's arms, once straight, are virtually inextensible and so it becomes impossible to lean any further to windward without also pulling the sail to windward. For small angles of tilt the reduction of propulsive component is negligible and the vertical (lift) component has the beneficial effect of reducing displacement and consequently also reducing the hydrodynamic resistance to motion. Nevertheless, for conventional sailboards, this effect is soon outweighed by the reduction of the propulsive component and by the time the sail is tilted to 45.degree. the loss is approximately 30%. (In the ultimate case, with the sail horizontal, it would generate lift and drag, but no propulsive component).